RF filters used in mobile communication bands are divided into low pass filters (LPFs), band pass filters (BPFs), high pass filters (HPFs), and band stop/reject filters (BSF/BRFs). Among them, the use of band pass filters and band reject filters has been increasing recently because of the emergence of many carriers and attempts to use limited frequency resources efficiently.
This is because the roles of band pass filters to select accurately only the desired frequency bands and of band reject filters to pass all frequency bands well except a certain frequency band are directly connected to the sound quality of calls in the contemporary RF mobile communication system wherein many frequencies are minutely divided for use. These principles of filters use frequencies are minutely divided for use. These principles of filters use resonance made by combinations of inductance (L) and capacitance (C). Therefore, band reject filters or band pass filters suitable for use can be embodied by diversely combining inductance components and capacitance components.
As shown in FIGS. 1 and 2, a mobile communication system (10) made pursuant to conventional technologies is equipped with multiple band pass filters (12-16) between its base station system (2) and antenna (4) to pass different frequency bands. The band pass filters (12-16) are parallel-connected with each other to pass different frequency bands (f1, f2, f3). Toward this end, the mobile communication system (10) requires characteristics matching between band pass filters (12-16).
To match the characteristics of the multiple band pass filters (12-16) parallel-connected with each other, the mobile communication system (10) requires a combination device and a split device (20, 30) that will combine or split RF signals at both ends. However, the mobile communication system (10) made pursuant to conventional technologies can hardly be embodied as actual filter products because characteristics matching between parallel-connected band pass filters (12-16) is very difficult.